An Agricultural Balancing Act:A look at the biological control consequences of crop domestication

By Lauren Hunt and Jessica Grant

Ecological theories and models that attempt to explain interactions between plant, herbivore, and predator are innumerable. But just how accurate can these theories be? How many factors can be feasibly fitted to a model without making it cumbersome? What major variables are missing from these interaction evaluations? Dr. Rodriguez-Saona cites one key aspect of agro-ecosystems that has often been ignored: anthropogenic effects of crop selection.In terms of biological control, there are numerous top-down and bottom-up factors that play a role in the dynamic balance between plants and herbivores. Top-down factors are regulating mechanisms in which herbivore population numbers are controlled through upper trophic level organisms such as predators, natural enemies and parasitoids. In response to a lack of mobility, plants have developed an array of bottom-up mechanisms with which they can resist attack. Bottom-up controls are synergistically balanced with top-down controls in natural systems for overall suppression of herbivore populations. Plants utilize bottom-up mechanisms directly, through chemical and physical features used to resist damage and indirectly, by signaling to predators of an ongoing attack. These cries for help are in the form of volatiles (herbivore-induced plant volatiles, or HIPVs) that predators and parasitoids can respond to, finding food and reproductive hosts while simultaneously defending the plant. The HIPVs influence both bottom-up and top-down controls in this tri-trophic interaction.

Dr. Rodriguez-Saona seeks to understand these tri-trophic interactions with managed agricultural systems. He stresses that meaningful progress in pest management must include evaluation of human domesticationas a factor influencing plant-mediated defense. Exploiting the availability of recently domesticated cranberry crop, its related varieties and its wildtype ancestor, Dr. Rodriguez-Saona was able to critically examine effects of domestication in a controlled setting. In a greenhouse study of five different cranberry varieties, Dr. Rodriguez-Saona applied a combination of jasmonic acid and gypsy moth caterpillars to investigate interactions between plant yield and the gypsy moth pest damage. He observed that the moth caterpillars did not perform as well on the jasmonic acid treated cranberries. This outcome was to be expected, as jasmonic acid is a signaling pathway that initiates plant defense in response to herbivore feeding. The five different cranberry varieties were then assessed for nutritional content, digestibility and HIPV production. He observed that bottom-up controls conferred through plant nutrition, digestibility, and HIPV production are altered by domestication as seen in the wild versus domesticated varieties (Rodriguez-Saona 2011b). Ultimately, bottom-up defense emanating from plants in hybridized/domesticated varieties were significantly reduced in the selection of higher yields with larger, more uniform fruit.

With the reduction of bottom-up controls in agricultural plants by domestication, growers need to increase top-down controls to supplement pest suppression. One proposed management tool to increase natural predators in agricultural fields is the use of PredaLure®. PredaLure is a broad range methyl salicylate (MeSA) synomone (a chemical produced by an individual of one species that benefits both the producer, i.e., plant, and the recipient which is of a different species, i.e., predator) lure that attracts beneficial predators over several weeks, similar to HIPVs. It is marketed to attract: lacewing, lady beetle, syrphid, minute pirate bug, and predatory mite species. All of these, with the exception of the predatory mites, which feed exclusively on spider mites, are generalist predators.

Possible mechanisms through which products such as Predalure, developed with synthetic MeSA, may attract natural enemies. Arrows represent the movement or attraction of the predator to the plant. Induction occurs when the plant exposed to the synthetic lure produces HIPVs. Priming occurs when neighboring plants receive a warning signal to induce volatile response. Habituation occurs when predators are over-exposed to a signal, such that the predator response is depleted (source: Kaplan 2012).

More research is required to determine the effectiveness of products such as PredaLure in controlling pest species throughout the growing season. Current gaps in determining the effectiveness of such lures lies in whether or not the predators disperse throughout the field (Rodriguez-Saona 2011a). A predator may be enticed to the field in the presence of this attractant, but only remain locally around the bait and therefore have limited biological control effects. Another question centers on the lure’s ability to provide constant pest suppression throughout the growing season. Each predator has varying temporal abundances and different consumption capacities. Maximizing differences in the predator communities may take more than just a lure to the field. In short, baits like PredaLure may need to work in conjunction with other natural enemy enhancing practices such as conservation strips to increase habitat for predators (Rodriguez-Saona 2011b).

The discussion of the interrelation between pest-predator dynamics, crop domestication and management to enhance top-down control in managed systems is a scintillating topic. For more information on the consequences of cranberry domestication or the response of predators to methyl salicylate lure check out a few of Dr. Rodriguez-Saona’s most recentpublications.

References:

Kaplan, I. 2012. Attracting carnivorous arthropods with plant volatiles: The future of biocontrol or playing with fire? Biological Control 60(2): 77–89.

About The Authors: Jessicais a master’s student in Dr. Lamp’s lab focusing in integrated pest management. She currently is working with kudzu bug’s cold tolerance in Maryland. For more on Jessica visit the Lamp Lab page. Lauren is a master’s student in Cerruti R.R. Hooks’ lab who is conducting research on sustainable agricultural practices. She is currently studying effects of habitat manipulation as a pest control tactic in organically managed cropping systems.